A conventional electrophotographic type image forming device is provided with a sheet supply roller (pick up roller). The sheet supply roller contacts an uppermost sheet of stacked sheets in a sheet supply tray in order to supply sheets to an image forming unit on a sheet-by-sheet basis. It is important for the sheet supply roller to be pressed against the sheet stack at a predetermined pressure in order to supply sheets to the image forming unit on a sheet-by-sheet basis. However, the sheets are consumed one after another as images are formed, and a height of the uppermost sheet is gradually lowered. Thus, in order to maintain pressure of the sheet supply roller against the sheet stack, there is a need to provide a mechanism for elevating the height of the uppermost sheet of the sheet stack or a mechanism for moving the sheet supply roller downward in association with consumption of the sheets.
Laid-open Japanese Patent Application Publication No. 2007-269462 discloses an image forming device in which a drive force transmission mechanism is provided to transmit a drive force to a stack plate (pressure plate) in a sheet supply tray via an acting member in order to elevate the stack plate. The drive force transmission mechanism has a one-way clutch for regulating the stack plate from being displaced downwardly. In this image forming device, the one-way clutch is employed to prevent the stack plate from moving downward due to a weight of recording sheets stacked on the stack plate even when the drive force is not transmitted to the stack plate.